fix: Changes Map.Sector.Items to link list. (#1547)

### Summary

Eliminates `IPooledEnumerable<T>` and `eable.Free()` from `Map` for items. This drastically simplifies code that iterates in range, for example:

```cs
foreach (var item in m.GetItemsInRange(5))
{
}
```
The code above no longer requires an eable and calling `Free()`.

Projects/UOContent/Spells/Seventh/GateTravel.cs

@@ -141,9 +141,7 @@ namespace Server.Spells.Seventh
 
         private static bool GateExistsAt(Map map, Point3D loc)
         {
-            var eable = map.GetItemsInRange(loc, 0);
-
-            foreach (var item in eable)
+            foreach (var item in map.GetItemsInRange(loc, 0))
             {
                 if (item is Moongate or PublicMoongate)
                 {